The present invention relates to an amplitude-modulated signal generator.
In the field of a radiocommunication, an amplitude-modulated signal generator is employed for the amplitude modulation of a carrier wave in accordance with a suitable information wave. A number of amplitude-modulated single generators have been proposed, and the ones which employ transistor are known as a base modulation type, an emitter modulation type and a collector modulation type, all of which need a transformer.
Since any one of these conventional amplitude-modulated signal generators requires the use of a transformer, the generator becomes bulky in size and it requires a high manufacturing cost. Particularly, when the conventional amplitude-modulated signal generator is used in an ultrasonic wave generator capable of generating an ultrasonic signal of relatively low frequency, for example, 30 to 500 KHz, the transformer must have a high inductance, resulting in a very large size generator. Furthermore, a precise adjustment is necessary.
On the other hand, an amplitude-modulated signal generator using no transformer is also well known. One amplitude-modulated generator of this type includes a circuit wherein an electric current generated from a constant current source defined by a differential amplifier is varied by a carrier wave to produce an amplitude modulated wave. Another known type is a product modulating circuit wherein an analog multiplier is utilized. However, these modulating circuits are complicated in circuit arrangement and, requires a high manufacturing cost, particularly when they are desired to be fabricated in a compact size.